Low birth weight is associated with cardiovascular disease and its risk factors in adulthood. However, information is limited regarding its impact on heart rate (HR), an established risk factor for cardiovascular disease. This study assessed the hypothesis that birth weight is associated with HR at rest at different ages. The study sample consisted of 6,282 black and white participants enrolled in the Bogalusa Heart Study, aged 4 to 52 years with a mean age of 19.4 years. HR data at rest were available in 2,344 children (4 to 11 years old), 1,622 adolescents (12 to 19 years old), and 2,316 adults (20 to 52 years old). Birth certificate records, including information on birth weight and gestational age, were obtained from the Louisiana State Office of Public Health. HR showed a significant decreasing trend with increasing age, with blacks having a lower slope than whites. In multivariable linear regression analyses, adjusted for age, race, gender, body mass index, and gestational age, the association between lower birth weight (kg) and increased HR (beats/min) was significant in adults (regression coefficient, β = −1.21, p = 0.006) but not significant in children (β = −0.31, p = 0.461) and adolescents (β = −0.72, p = 0.157). The association did not differ significantly between races. The birth weight-HR association did not change markedly in the models without adjustment for body mass index. In conclusion, these results suggest that the association of prenatal growth retardation with increased cardiovascular disease risk in later life might be partly through its relation with HR at rest.
Low birth weight at full term, an indicator of intrauterine growth restriction, has been associated with adult cardiovascular disease and its risk factors. Among traditional risk factors, elevated blood pressure has been shown to be most strongly associated with low birth weight, and the effect size is amplified with increasing age. Increased heart rate (HR) at rest, another important hemodynamic parameter and well-established independent risk factor for cardiovascular disease, is associated with low birth weight in children and adults. However, data on the birth weight-HR association are still limited, especially the age-related trend in the strength of the association. The present study aimed to examine the association between birth weight and HR at rest in children, adolescents, and adults in black and white populations.
Methods
The Bogalusa Heart Study is a series of long-term studies in a semi-rural biracial (65% white and 35% black) community in Bogalusa, Louisiana, begun in 1973 by Dr. Gerald Berenson, focusing on the early natural history of cardiovascular disease since childhood. Cross-sectional surveys of children and adolescents aged 4 to 19 years and surveys of adults aged 20 to 52 years were conducted for cardiovascular risk factors in 1980 to 1994 and 1982 to 2010, respectively. Birth weight records of the participants were obtained in 2005 from the Louisiana State Public Health Office. Exclusion criteria included gestational age <37 weeks or >42 weeks of pregnancy, birth weight >4.5 kg or HR >150 beats/min at rest. After exclusion, 2,344 children aged 4 to 11 years (59% white and 50% male), 1,622 adolescents aged 12 to 19 years (57% white and 55% male), and 2,316 adults aged 20 to 52 years (67% white and 45% male) formed the study sample.
All subjects in this study gave informed consent at each examination. For those <18 years, consent of a parent/guardian was obtained. Study protocols were approved by the Institutional Review Board of the Tulane University Health Science Center.
Birth weight information on the Bogalusa Heart Study participants has been obtained from the Louisiana State birth certificates maintained by the Louisiana State Office of Public Health. Information includes birth weight, gestational age, year of birth, and parents’ age at birth.
Examinations of children and adults followed the same protocols. Height and weight were measured twice to ±0.1 cm and to ±0.1 kg, respectively. Body mass index (BMI, weight in kilograms divided by the square of the height in meters) was used as a measure of overall adiposity. HR at rest was counted at the radial pulse in a relaxed, sitting position. After at least 5-minute resting, 3 counts were made by 2 trained research members, and average of the 6 values was used.
Analysis of covariance was performed using general linear models to test differences in continuous variables in blacks versus whites and in men versus women. The differences in categorical variables were tested by the chi-square test. The rate of fetal growth was calculated as birth weight (kg)/gestational age (week) for each individual by race-sex group and then multiplied by the mean value of gestational age of the sample to convert into the original scale. Thus, the adjusted birth weight represented the rate of growth independent of the length of gestation period and was used in all subsequent analyses. To eliminate the bias due to correlations between repeated measurements, for individuals who had multiple measurements of HR in the same age period, the first measurement in childhood and the last in adolescence and adulthood were selected for analysis. The relation between HR and birth weight was examined by multiple linear regression models, adjusting for age, gender, and BMI by race group and in the total sample (with additional adjustment for race). For categorical analyses, quartiles of gestational age-adjusted birth weight were defined using cut-off points in race-sex groups. Covariates-adjusted mean values of HR were calculated by general linear model and used for trend analysis of HR by quartile of birth weight. Statistical Analyses were performed using SAS, version 9.3 (SAS Institute, Inc., Cary, North Carolina).
Results
Table 1 lists mean values and SD of study variables by race, gender, and age group. Whites and women had faster HR than blacks and men, respectively, in all age groups, except that there was no race difference in adults in both men and women. Whites had greater birth weight (both adjusted and unadjusted) and gestational age than blacks; men had greater birth weight than women.
| Variable | White | Black | Race Difference | |||
|---|---|---|---|---|---|---|
| Males | Females | Males | Females | Males | Females | |
| Childhood (4-11 years) | N=675 | N=698 | N=473 | N=498 | ||
| Age (year) | 9.0±1.7 | 8.9±1.7 | 8.8±1.9 | 8.7±1.9 | 0.020 | 0.111 |
| BMI (kg/m 2 ) | 18.0±3.4 | 17.9±3.6 | 17.7±3.6 | 17.8±3.6 | 0.391 | 0.763 |
| HR (beats/min) | 86.4±10.3 | 89.3±10.3 ∗∗ | 82.0±10.1 | 85.7±10.5 ∗∗ | <0.001 | <0.001 |
| Adolescence (12-19 years) | N=511 | N=418 | N=383 | N=310 | ||
| Age (year) | 14.7±2.0 | 14.8±2.0 | 15.7±2.2 | 15.2±2.1 ∗∗ | <0.001 | 0.010 |
| BMI (kg/m 2 ) | 22.5±5.0 | 22.2±4.8 | 22.6±5.0 | 23.1±5.4 | 0.279 | 0.052 |
| HR (beats/min) | 78.0±10.7 | 81.7±11.2 ∗∗ | 71.8±9.7 | 78.9±10.4 ∗∗ | <0.001 | 0.004 |
| Adulthood (20-52 years) | N=714 | N=831 | N=322 | N=449 | ||
| Age (year) | 33.9±9.1 | 33.3±9.0 | 32.3±9.4 | 32.4±9.3 | 0.007 | 0.079 |
| BMI (kg/m 2 ) | 28.3±6.1 | 27.2±7.6 ∗∗ | 27.2±6.7 | 29.8±8.3 ∗∗ | 0.065 | <0.001 |
| HR (beats/min) | 69.0±9.1 | 73.5±9.8 ∗∗ | 68.0±9.6 | 73.0±9.6 ∗∗ | 0.147 | 0.253 |
| Birth registry information | N=1900 | N=1947 | N=1178 | N=1257 | ||
| Gestational age (week) | 39.80±0.82 | 39.81±0.89 | 39.66±0.88 | 39.69±0.90 | <0.001 | <0.001 |
| Birth weight (kg) | 3.46±0.48 | 3.33±0.47 ∗∗ | 3.19±0.51 | 3.09±0.46 ∗∗ | <0.001 | <0.001 |
| Birth weight (kg) ∗ | 3.43±0.47 | 3.31±0.46 ∗∗ | 3.17±0.50 | 3.07±0.45 ∗∗ | <0.001 | <0.001 |
Figure 1 illustrates the relation between age and HR by race group. HR decreases significantly with increasing age in both blacks and whites. Despite the higher childhood HR in whites than in blacks, it decreased faster in whites as age increased, with p <0.001 for difference in slopes between race groups.
Birth weight tended to be inversely associated with HR across all race-age groups except for black children, after adjusting for covariates (including BMI) in multiple regression models ( Table 2 ). The regression coefficients did not differ significantly in blacks and whites (p = 0.479 in children, p = 0.604 in adolescents, and p = 0.958 in adults for differences in regression coefficients between blacks and whites). In the total sample, only adults showed significant association between birth weight and HR, although children and adolescents showed similar trends (inverse associations). The regression coefficients in Table 2 had only slight changes without BMI adjustment ( Table 3 ), which is consistent with that HR and BMI were not significantly correlated in children (r = 0.028, p = 0.394), adolescents (r = −0.021, p = 0.397), and adults (r = 0.023, p = 0.261) with no black-white differences.
| White | Black | Total | ||||
|---|---|---|---|---|---|---|
| β | P Value | β | P Value | β | P Value | |
| Childhood (n=2344) | ||||||
| Black race | -4.22 | <0.001 | ||||
| Age | -1.19 | <0.001 | -1.20 | <0.001 | -1.19 | <0.001 |
| Female sex | 2.72 | <0.001 | 3.65 | <0.001 | 3.11 | <0.001 |
| BMI | 0.06 | 0.477 | 0.06 | 0.549 | 0.06 | 0.339 |
| Birth weight ∗ | -0.60 | 0.279 | 0.10 | 0.881 | -0.31 | 0.461 |
| Adolescence (n=1622) | ||||||
| Black race | -3.80 | <0.001 | ||||
| Age | -1.60 | <0.001 | -1.33 | <0.001 | -1.50 | 0.119 |
| Female sex | 3.64 | <0.001 | 6.34 | <0.001 | 4.76 | <0.001 |
| BMI | -0.06 | 0.378 | -0.02 | 0.831 | -0.03 | 0.509 |
| Birth weight ∗ | -0.63 | 0.356 | -0.85 | 0.256 | -0.72 | 0.157 |
| Adulthood (n=2316) | ||||||
| Black race | -1.13 | 0.009 | ||||
| Age | -0.12 | <0.001 | -0.03 | 0.372 | -0.09 | <0.001 |
| Female sex | 4.29 | <0.001 | 4.85 | <0.001 | 4.50 | <0.001 |
| BMI | 0.03 | 0.430 | 0.05 | 0.333 | 0.04 | 0.180 |
| Birth weight ∗ | -1.19 | 0.028 | -1.17 | 0.131 | -1.21 | 0.006 |
| White | Black | Total | ||||
|---|---|---|---|---|---|---|
| β | P Value | β | P Value | β | P Value | |
| Childhood (n=2344) | ||||||
| Black race | -4.21 | <0.001 | ||||
| Age | -1.16 | <0.001 | -1.17 | <0.001 | -1.16 | <0.001 |
| Female sex | 2.73 | <0.001 | 3.67 | <0.001 | 3.11 | <0.001 |
| Birth weight ∗ | -0.55 | 0.320 | 0.17 | 0.792 | -0.25 | 0.552 |
| Adolescence (n=1622) | ||||||
| Black race | -3.81 | <0.001 | ||||
| Age | -1.63 | <0.001 | -1.34 | <0.001 | -1.51 | <0.001 |
| Female sex | 3.65 | <0.001 | 6.32 | <0.001 | 4.75 | <0.001 |
| Birth weight ∗ | -0.72 | 0.292 | -0.87 | 0.243 | -0.76 | 0.130 |
| Adulthood (n=2316) | ||||||
| Black race | -1.07 | 0.013 | ||||
| Age | -0.11 | <0.001 | -0.02 | 0.527 | -0.08 | <0.001 |
| Female sex | 4.27 | <0.001 | 4.97 | <0.001 | 4.51 | <0.001 |
| Birth weight ∗ | -1.17 | 0.031 | -1.06 | 0.164 | -1.16 | 0.008 |
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